We have so far failed to find a good way to determine the stack base.
It is highly recommended that GC_stackbottom be set explicitly on program
startup.  The supplied value sometimes causes failure under AIX 4.1, though
it appears to work under 3.X.  HEURISTIC2 seems to work under 4.1, but
involves a substantial performance penalty, and will fail if there is
no limit on stack size.
